To contents page A-level and Undergraduate Bridging between Proton NMR Created by Chris Phillips whilst a final year MChem student in the Department of Chemistry at the

Click on the topic you wish for information on: Magnetic resonance Spectra Splitting patterns Interpreting spectra quiz Shielding Equipment Change page Return to contents Text More information on that term Embedded videos require an up to date version of Flash and Activex plugin to function correctly

Magnetic resonance

One of the main features of NMR is the use of magnetic fields Moving charges produce their own magnetic fields. This means nuclei have a field. Nuclei align in a magnetic field, in a similar way to bar magnets aligning when next to other magnets. You would have to apply force to move the magnet against the fields of the other magnets, making the alignment shown below, the most stable alignment Without the other field, the magnet can lie in any direction N S External magnetic field S N

A nucleus’ alignment in a field is described through the quantum property named ‘spin’. The quantum property of spin can be represented by saying a nucleus can align in 2 directions (‘up’ and ‘down’), as a bar magnet can in another magnetic field. For protons, these 2 directions or spin states are described by quantum numbers +1/2 and -1/2. Without a field, the spin state does not matter, they are equally energetically favourable, like a bar magnet without other magnets nearby. +1/2-1/2 ‘up’ ‘down’ N S Direction of field External magnetic field

When a magnetic field is applied (B 0 ), one spin state becomes higher in energy, and one lower States are separated in energy by an amount, ΔE The fields of the nuclei align with or against the applied magnetic field The stronger the field, the greater the energy gap between ‘up’ and ‘down’ states Larger separation means more nuclei are in the lower energy state ‘Magnetic resonance’ is using radiowaves to swap nuclei between different spin states; changing the alignment of nuclei within the field +1/2 -1/2 B0B0 ΔE ‘Against’ is higher in energy ‘With’ is lower in energy N S

In NMR, radiofrequency pulses can change alignments to an excited energy state, the higher energy state The frequency required is unique to the particular energy gap, depending on a proton’s environment After a pulse, protons release energy as they settle back to their natural state; this is relaxation Radiofrequency emissions during relaxation are recorded against time A mathematical process, called a Fourier Transformation, converts the signal into the NMR spectrum How does NMR use spin?

zk-nV06g The principles of magnetic resonance, from the perspective of MRI:

1. Apply magnetic field 2. Begin pulses 3. Stop pulses Outside of a field – random orientations Field applied – separates spin up and down states Try your own magnetic resonance experiment! Fourier transformation Detector Radiofrequency pulse excites nuclei so they can spin flip Excited spin state relaxes to original alignment. Radiofrequency emitted. 4. Restart B0B0 B0B0 Population of energy levels

1. Apply magnetic field 2. Begin pulses 3. Stop pulses Outside of a field – random orientations Field applied – separates spin up and down states Try your own magnetic resonance experiment! Fourier transformation Detector Radiofrequency pulse excites nuclei so they can spin flip Excited spin state relaxes to original alignment. Radiofrequency emitted. B0B0 B0B0 Population of energy levels

Equipment

The main principle of NMR, is the use of A field is applied to a proton, which causes alignment with, or against, the magnetic field A radiofrequency pulse, specific to the nucleus involved, ‘flips’ the alignment Radiofrequency emissions are recorded, and are unique to the nucleus being observed. Magnetic resonance is the excitation of nuclei using radiofrequencies while a magnetic field is applied x magnetic resonance

How to run an NMR experiment:

Click on a label for more information: Radiowave generator Radiowave generator Detector Magnetic field (B 0 ) Magnet Produces pulses of radiowaves which change the alignment of protons Generates applied magnetic field which creates an energy difference in spin states and causes alignment More The detector picks up energy emitted as protons relax Signal is then converted into NMR spectrum for analysis, using Fourier Transformation A Basic NMR schematic The magnets are superconductors, cooled to 4K (as close as possible), and submerged in liquid helium

Fourier Transformation The Fourier Transformation is a mathematical function It converts the signal produced by relaxing nuclei into the NMR spectrum which is analysed The time based signal is converted into a frequency based signal More environments result in a more complicated signal as characteristic frequencies superimpose Chemical shift/ ppm Time / s

Spectra

Each peak (or ) represents a unique proton environment Each peak (or ) represents a unique proton environment An environment is a group of equivalent protons in a molecule. An environment is a group of equivalent protons in a molecule. Equivalent protons are identical All equivalent protons give the same signal All equivalent protons give the same signal Multiplets are groups of peaks on a spectrum which collectively represent a single proton environment Here, there are 2 proton environments: a CH 3 group, and a CH 2 group multiplet x

The spectra have 'chemical shift' along the x-axis Chemical shift is related to a proton’s resonant frequency and gives information about a proton's environment, such as the electronegativity of neighbouring atoms Intensity, on the y-axis, is rarely marked on NMR spectra, but gives information on the number of protons within an environment

In order to produce a value for chemical shift, tetramethylsilane (TMS) is used TMS is a standard, assigned a chemical shift of 0 All other signals are therefore compared to TMS Why is TMS used? This works by comparing the frequency required for resonance in the observed environment with the resonant frequency of TMS Chemical shift is therefore a frequency value, relative to the standard TMS has the following key properties: - 12 hydrogens, all identical, provides a very clear, strong signal to compare against - Protons in TMS’s C-H bonds have the greatest electron cloud of almost all other C-H bonds, ensuring all other signals are to the left of the TMS signal TMS has the following key properties: - 12 hydrogens, all identical, provides a very clear, strong signal to compare against - Protons in TMS’s C-H bonds have the greatest electron cloud of almost all other C-H bonds, ensuring all other signals are to the left of the TMS signal x

NMR spectra have a 'downfield' and an 'upfield'. As a peak's chemical shift increases in value, it moves downfield, having been As a peak's chemical shift increases in value, it moves downfield, having been DownfieldUpfield Did you know? Deshielding is the reduction of a proton’s electron cloud. One example of this is a neighbouring electronegative atom withdrawing electron density. As a signal is found further downfield, it means the electron cloud around the proton is less than the cloud around protons in TMS. Deshielding is the reduction of a proton’s electron cloud. One example of this is a neighbouring electronegative atom withdrawing electron density. As a signal is found further downfield, it means the electron cloud around the proton is less than the cloud around protons in TMS. x deshielded Some protons are so well shielded, that they are more heavily shielded than TMS. This gives them a negative shift. The protons labelled here are shielded by the delocalised electrons opposing the magnetic field. Some protons are so well shielded, that they are more heavily shielded than TMS. This gives them a negative shift. The protons labelled here are shielded by the delocalised electrons opposing the magnetic field. x

Proton NMR gives the number of hydrogen atoms present The size of the peaks gives the relative number of protons in each environment When a peak has been split, the area under a group of peaks is taken. This is an integrated value Therefore, the peak will give the correct number of protons within the environment, even if the intensity has been reduced by being split into a multiplet

The values don't always have to be integers, the relative sizes are what's important! These values can sometimes be found in several locations, but are always near the peak they are assigned to This may be: Beside a line indicating which peaks have been included Below the peak On a ‘normalised’ y-axis Click here for some alternate peak values. Notice they follow the same 1:2:1 ratio between the peaks Click here for some alternate peak values. Notice they follow the same 1:2:1 ratio between the peaks

Shielding

Shielding determines how far, or, a peak is shifted As a proton becomes deshielded, it is shifted further downfield, as the magnetic field is able to affect it more Shielding determines how nuclei interact with the magnetic field and the of the signal The electron cloud surrounding a nucleus opposes the applied field The more electrons, the greater the shielding around a proton, so the field is more strongly opposed Downfield is an increasing chemical shift, as a proton is more deshielded downfield upfield Upfield is the decreasing chemical shift, as a proton is more shielded x x chemical shift The resonant frequency of a proton, compared to the standard in NMR of tetramethylsilane (TMS) See section: Spectra The resonant frequency of a proton, compared to the standard in NMR of tetramethylsilane (TMS) See section: Spectra x

Electronegative atoms (eg. oxygen or chlorine) will draw electron density away from the protons, deshielding them. Less electronegative atoms like carbon will not pull electron density, leaving the proton shielded, so it is not shifted. How does an electronegative atom affect the cloud? Click to see How much is a weakly withdrawing atom going to affect the cloud? Click to see The electron cloud is pulled away from the proton The electron cloud is left mostly unchanged

As the proton’s electron cloud reduces, the nucleus is exposed to more of the external magnetic field (B 0 ) The energy gap increases causing magnetic resonance frequency to change. This changes the emissions frequency from relaxation This results in a greater chemical shift and the signal appears further downfield DownfieldUpfield Click to see how far each proton will be shifted

Signals tend to fall within particular ranges, depending on the environment the proton is in This helps identify the signal based on the chemical shift

Splitting patterns

In the spectrum below, there is a signal for each environment, however, they appear split. These split peaks are multiplets. The number of peaks in a multiplet provides information about neighbouring protons.

Splitting is caused by the interaction between inequivalent protons, called coupling Proton-proton coupling usually takes place 3 bonds away from each other The number of peaks, or multiplicity, comes from the number of protons interacting

The number of peaks follows an n+1 pattern, where n is the number of protons in the interacting environment The number of peaks observed is the multiplicity The intensity of the peaks in a multiplet follows the pattern of Pascal’s triangle eg. If a group of protons has 2 neighbouring equivalent protons, it will be split into a triplet. Singlet Triplet Doublet Quartet 0 neighbouring protons 1 neighbouring proton 2 neighbouring protons 3 neighbouring protons

The CH 3 is coupled with the CH 2. There are 2 protons, so CH 3 is split into a triplet The CH 2 is coupled with the CH 3. There are 3 protons and therefore split CH 2 into a quadruplet Intensity = 1:3:3:1 Intensity = 1:2:1 Click on the group to see how it will appear on the spectrum:

Origin of the Pascal’s triangle pattern Starting simple: doublets CC H H Observing the signal of H, it will be split by H H interacts with H's magnetic field, H 0 The magnetic field interactions cause splitting between higher and lower energy levels H0H0

There are two possible interactions that H's field can have with H's field (H 0 ) One possibility will be with, and one against the interacting field As these states are interacting with another field, the states have different energies CC H H There is a 1:1 ratio distributed between these levels This gives the doublet 1:1 H0H0 Down (Against) Up (With)

As a more complicated example: quartet This time, H interacts with H 3 – three equivalent protons Each of the spins of the three protons can be aligned with or against H 0 Due to the interaction being between different magnetic fields, the different possible alignments have different energies, some of them are equivalent CC H H3H3 H0H0 1 3 of equal energy 1 4 (n+1) peaks 1:3:3:1 intensity pattern Quartet splitting pattern All with All against 2 with, 1 against 1 with, 2 against Complete alignment ‘with’ is the most favourable, so is lowest in energy The next most favourable is 2 with. But there are 3 possible combinations which give 2 ‘with’ alignments and 1 ‘against’. These are equal in energy.

Interpreting spectra quiz (You may want pen and paper for this!) With each structure, identify the correct NMR Click on the circle to confirm your answer

? ? ?

Good try, but have another go and see if you can work out the correct spectrum Return to question Try this: Identify the equivalent protons and their environments Look to see which environment is coupling with others, how many hydrogens is it coupling to (remember: n+1)? Check to see that the integrated intensity matches the group on the molecule

Well done! Click for the next question You should have noticed that: There are two groups of equivalent protons; the single protons, and the methyl groups The single proton split the methyl signal into a doublet The methyl group split the single proton signal into a quartet The single proton signal is closer to the double bond, making the signal more downfield

Note: In solvent, due to proton exchange, the OH group does not couple ? ? ?

Good try, but have another go and see if you can work out the correct spectrum Try this: Work out how many different environments there are Look to see which environment is coupling with others, how many hydrogens is it coupling to (remember: n+1)? Check to see that the integrated intensity matches the group on the molecule Return to question

Well done! Click for the next question You should have noticed that: There are three environments; the methyl group, the OH group, and the pair of protons on the centre carbon The methyl group will be split into a triplet by the pair or centre protons The pair of protons will give a signal which is split into a quartet by coupling with the methyl group

Note: TMS reference signal occurs at a shift of 0 Hz when present ? ? ?

Good try, but have another go and see if you can work out the correct spectrum Try this: With the integrated peaks, remember the ratio of number of protons is provided, not an absolute number of protons. Try changing the numbers while maintaining the ratio. Consider likely chemical shifts for the signals and see how it compares with what is present Return to question

Well done! Click for the end the quiz You should have noticed that: There are three environments, excluding the TMS’s signal at 0 ppm. The ratio of peak intensities can be easily simplified to 3:2:3, fitting the number of protons in the molecule. The pair of protons will be more downfield due to being closer to the ester functional group, than the methyl group it couples with

Congratulations! This is the end of the quiz Return to the contents page You can find some more advanced problems through the following link: